1. Field of the Invention
The invention relates to a method and apparatus for detecting a watermark embedded in a suspect image.
2. Description of the Related Art
Watermarking is a technique to certify the ownership of images or video. Usually, the watermark is embedded by adding a specific low-amplitude noisy pattern to the image. The noisy pattern represents the watermark. Whether or not a suspect image has an embedded given watermark is detected at the receiver end by computing the correlation of the suspect image with an applied version of said watermark, and comparing the correlation with a threshold. If the correlation is larger than the threshold, the applied watermark is said to be present, otherwise, it is said to be absent.
Applicant""s previously filed International Patent Application IB99/00358 corresponding to U.S. patent application Ser. No. 09/423,276, filed Nov. 4, 1999 (PHN 17.316), discloses an arrangement for detecting a watermark that is embedded by repeating a small-sized basic watermark pattern over the extent of the image. Such a xe2x80x9ctilingxe2x80x9d operation allows the watermark detection process to search the watermark over a relatively small space, and improves the reliability of detection.
It is known that most watermarking techniques are not resistant to geometric distortions of the image. Manipulations, such as translation, scaling, rotation, or stretching, destroy the correlation between the manipulated image and the applied watermark. The above-mentioned prior-art watermark detector is resistant to translation but lacks the ability of detecting the watermark if the image has been scaled, rotated or stretched.
It is an object of the invention to provide an improved watermark detection method and apparatus.
To this end, the method of detecting a watermark in a suspect image comprises the steps of detecting whether said suspect image includes a periodically repeated embedded data pattern, and concluding that said periodically repeated data pattern represents an embedded watermark. The invention is based on the recognition that operations, such as scaling, rotating and stretching, change but do not destroy the periodicity of a watermark if said watermark is embedded by means of the above-mentioned xe2x80x9ctilingxe2x80x9d operation. Accordingly, the mere presence of a periodically repeated data pattern in the suspect image signifies that the image has been watermarked.
Detection as to whether the embedded watermark is a specific given watermark is achieved by processing the suspect image or the given watermark in such a way that the original correlation is restored. This is achieved in an embodiment of the method which comprises the steps of determining the periodicity of said data pattern, applying a given watermark having a given periodicity, processing the suspect image and/or the given watermark so as to match the periodicity of the data pattern in the processed suspect image with the periodicity of the processed given watermark, and detecting whether the data pattern in the processed suspect image corresponds to the processed given watermark. The aim of the step of processing the suspect image is to undo the manipulation (scaling, rotation, stretching) which the suspect image has undergone after it is watermarked.
U.S. Pat. No. 5,636,292 discloses the step of adding a separate calibration signal (e.g., a sine wave with a specified frequency) to the image. When the image is scaled or rotated, the frequency of the sine wave changes, which results in a peak displacement in the image""s frequency spectrum. The invention differs from this prior art in that the periodic watermark pattern itself provides the calibration parameters.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.